An electrical relay is an electrically operated switch that consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. Relays are used to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. They are used wherever it is necessary to control a high power or high voltage circuit with a low power circuit, especially when galvanic isolation is desirable. Relays were first used in long-distance telegraph circuits as signal repeaters, and were used extensively in telephone exchanges and early computers to perform logical operations.

Relays can be divided into two main categories: movable contacts (mechanical relay) and no movable contacts (MOS FET relay, solid-state relay) . Mechanical relays have moving contacts, while solid-state relays employ semiconductor and electrical switching elements such as triac and MOS FET. By the operation of these electronic circuits, signals, currents, and voltages are switched ON or OFF electronically.

Relays are used for a wide range of applications such as home automation, cars and bikes (automobiles), industrial applications, DIY Projects, test and measurement equipment, and many more. They can be used in the place of other forms of switches, or they can be specifically designed based on factors such as required amperage. Relays are highly versatile components that are just as effective in complex circuits as in simple ones.